Point defects in synthetic diamond material, particularly quantum spin defects and/or optically active defects, have been proposed for use in various sensing, detecting, and quantum processing applications including: magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing devices such as for quantum computing.
Many point defects have been studied in synthetic diamond material including: silicon containing defects such as silicon-vacancy defects (Si-V), silicon di-vacancy defects (Si-V2), silicon-vacancy-hydrogen defects (Si-V:H), silicon di-vacancy hydrogen defects (S-V2:H); nickel containing defect; chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N-V), di-nitrogen vacancy defects (N-V-N), and nitrogen-vacancy-hydrogen defects (N-V-H). These defects are typically found in a neutral charge state or in a negative charge state. It will be noted that these point defects extend over more than one crystal lattice point. The term point defect as used herein is intended to encompass such defects but not include larger cluster defects, such as those extending over ten or more lattice points, or extended defects such as dislocations which may extend over many lattice points.
It has been found that certain defects are particularly useful for sensing, detecting, and quantum processing applications when in their negative charge state. For example, the negatively charged nitrogen-vacancy defect (NV−) in synthetic diamond material has attracted a lot of interest as a useful quantum spin defect because it has several desirable features including:                (i) Its electron spin states can be coherently manipulated with high fidelity owing to an extremely long coherence time (which may be quantified and compared using the transverse relaxation time T2 and/or T2*);        (ii) Its electronic structure allows the defect to be optically pumped into its electronic ground state allowing such defects to be placed into a specific electronic spin state even at non-cryogenic temperatures. This can negate the requirement for expensive and bulky cryogenic cooling apparatus for certain applications where miniaturization is desired. Furthermore, the defect can function as a source of photons which all have the same spin state; and        (iii) Its electronic structure comprises emissive and non-emissive electron spin states which allows the electron spin state of the defect to be read out through photons. This is convenient for reading out information from synthetic diamond material used in sensing applications such as magnetometry, spin resonance spectroscopy and imaging. Furthermore, it is a key ingredient towards using the NV− defects as qubits for long-distance quantum communications and scalable quantum computation. Such results make the NV− defect a competitive candidate for solid-state quantum information processing (QIP).        
The NV− defect in diamond consists of a substitutional nitrogen atom adjacent to a carbon vacancy as shown in FIG. 1a. Its two unpaired electrons form a spin triplet in the electronic ground state (3A), the degenerate ms=±1 sublevels being separated from the ms=0 level by 2.87 GHz. The electronic structure of the NV− defect is illustrated in FIG. 1b from Steingert et al. “High sensitivity magnetic imaging using an array of spins in diamond”, Review of Scientific Instruments 81, 043705 (2010). The ms=0 sublevel exhibits a high fluorescence rate when optically pumped. In contrast, when the defect is excited in the ms=±1 levels, it exhibits a higher probability to cross over to the non-radiative singlet state (1A) followed by a subsequent relaxation into ms=0. As a result, the spin state can be optically read out, the ms=0 state being “bright” and the ms=±1 states being dark. When an external magnetic field is applied, the degeneracy of the spin sublevels ms=±1 is broken via Zeeman splitting. This causes the resonance lines to split depending on the applied magnetic field magnitude and its direction. This dependency can be used for vector magnetometry as the resonant spin transitions can be probed by sweeping the microwave (MW) frequency resulting in characteristic dips in the optically detected magnetic resonance (ODMR) spectrum as shown in FIG. 2a from Steinert et al.
Steinert et al. employed ion implantation to create a homogenous layer of negatively charged NV− centres into an ultrapure {100} type IIa diamond. The ensemble NV− sensor was found to offer a higher magnetic sensitivity due to the amplified fluorescence signal from a plurality of sensing spins. Another option is vector reconstruction since the diamond lattice imposes four distinct tetrahedral NV− orientations as shown in FIG. 2b from Steinert et al. The magnetic field projections along each of these axes can be measured as a single composite spectrum and a numerical algorithm used to reconstruct the full magnetic field vector. The magnitude (B) and orientation (θB, φB) of the external magnetic field can be calculated by analyzing the ODMR spectra based on an unconstrained least-square algorithm.
One major problem in producing materials suitable for quantum applications is preventing quantum spin defects from decohering, or at least lengthening the time a system takes to decohere (i.e. lengthening the “decoherence time”). A long decoherence time is desirable in applications such as quantum computing as it allows more time for the operation of an array of quantum gates and thus allows more complex quantum computations to be performed. A long decoherence time is also desirable for increasing sensitivity to changes in the electric and magnetic environment in sensing applications.
WO 2010010344 discloses that single crystal synthetic CVD diamond material which has a high chemical purity, i.e. a low nitrogen content, and wherein a surface of the diamond material has been processed to minimise the presence of crystal defects, can be used to form a solid state system comprising a quantum spin defect. Where such materials are used as a host for quantum spin defects, long decoherence times are obtained at room temperature and the frequency of the optical transitions used to read/write to devices are stable.
WO 2010010352 discloses that by carefully controlling the conditions under which single crystal synthetic CVD diamond material is prepared, it is possible to provide synthetic diamond material which combines a very high chemical purity with a very high isotopic purity. By controlling both the chemical purity and the isotopic purity of the materials used in the CVD process, it is possible to obtain synthetic diamond material which is particularly suitable for use as a host for a quantum spin defect. Where such materials are used as a host for quantum spin defects, long decoherence times are obtained at room temperature and the frequency of the optical transitions used to read/write to the devices are stable. A layer of synthetic diamond material is disclosed which has a low nitrogen concentration and a low concentration of 13C. The layer of synthetic diamond material has very low impurity levels and very low associated point defect levels. In addition, the layer of synthetic diamond material has a low dislocation density, low strain, and vacancy and self-interstitial concentrations which are sufficiently close to thermodynamic values associated with the growth temperature that its optical absorption is essentially that of a perfect diamond lattice.
In light of the above, it is evident that WO 2010010344 and WO 2010010352 disclose methods of manufacturing high quality “quantum grade” single crystal synthetic CVD diamond material. The term “quantum grade” is used herein for diamond material which is suitable for use in applications that utilize the material's quantum spin properties. Specifically, the quantum grade diamond material's high purity makes it possible to isolate single defect centres using optical techniques known to the person skilled in the art. The term “quantum diamond material” is also used to refer to such material.
One problem with quantum materials is that single photon emission from quantum spin defects in such materials can be very weak. For example, NV− defects in diamond exhibit a broad spectral emission associated with a Debye-Waller factor of the order of 0.05, even at low temperature. Emission of single photons in the Zero-Phonon Line (ZPL) is then extremely weak, typically of the order of a few thousands of photons per second. Such counting rates might be insufficient for the realization of advanced QIP protocols based on coupling between spin states and optical transitions within reasonable data acquisition times.
The problem of weak emission may be alleviated to some extent by increasing the number of quantum spin defects such that a large number of emitting species exists in the material. To form negatively charged defects requires an electron donor such as a nitrogen or phosphorous. Accordingly, to increase the number of negatively charged defects one could increase the concentration of electron donors within the material. However, such electron donors may undergo dipole coupling with the negatively charged quantum spin defects lowering the decoherence time of the negatively charge quantum spin defects. Accordingly, the problem to be solved becomes how to increase the number of negatively charged quantum spin defects while not unduly lowering the decoherence time of the negatively charged quantum spin defects. Alternatively, for certain applications it may be desirable to have relatively few negatively charged quantum spin defects but where each negatively charged quantum spin defect has a very high decoherence time. The problem then is how to form a negatively charged quantum spin defect while ensuring that the electron donor required to form the defect does not unduly lower the decoherence time.
It is an aim of certain embodiments of the present invention to at least partially solve one or more of the aforementioned problems.